The present invention relates to a driver circuit for a semiconductor light-emitting element such as a light-emitting diode (LED) and a light fixture including the driver circuit.
U.S. Pat. No. 7,071,762 discloses an LED light fixture that uses a switching power supply to receive power from a DC power source and supply a DC current to an LED. Dimming of the LED is controlled according to a burst dimming control method to intermittently interrupt high frequency operation of the switching power source based on a low frequency dimming control. U.S. Pat. No. 7,071,762 further discloses that the low frequency dimming control is feedback controlled based on the current flowing to the LED.
According to a technique described in U.S. Pat. No. 7,071,762, on the assumption that the switching power supply is operated in a continuous mode, to prevent magnetic saturation of an inductor of the switching power supply, the controller of the switching power supply restricts the peak value of current flowing to the inductor. In contrast, by adopting a discontinuous mode of operation, the driver circuit can be simplified. In the discontinuous mode of operation, the switching element of the driver circuit is turned on after a period of no current flowing through an inductor of the driver circuit. Moreover, by setting the OFF time of the switching element to be much longer than the ON time of the switching element, dimmed lighting can be stably realized at very low luminance output (U.S. Patent Application Pub. No. 2011/000457). Although dimmed lighting in a low luminance range can be achieved relatively easily in the discontinuous mode, luminance output varies largely during discontinuous operation in medium to high luminance range due to variation in load characteristics caused by heat generated from the semiconductor light-emitting element (e.g., an LED).